box: move replicaset.vclock into instance_vclock

Storing vclock of the instance in replicaset.vclock wasn't right.
It wasn't vclock of the whole replicaset. It was local to this
instance. There is no such thing as "replicaset vclock".

The patch moves it to box.h/cc.

Part of #10113

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit f1e8e4e1)